The present invention relates to a tracking error signal detector that is used in an optical disk apparatus for recording/reproducing data optically.
In recent years, the applicability of optical disk apparatuses for recording/reproducing information has increased significantly. An example of a conventional tracking error signal detector used in the optical disk apparatuses will be described below by referring to the drawings.
FIG. 17 shows the configuration of an optical disk apparatus that includes a conventional tracking error signal detector. The tracking error signal detector includes an amplifier 1704, a PLL circuit 1705, a timing generation circuit 1706, and a sample-hold (S-H) circuit 1707. Reference numeral 1708 denotes a tracking control device that drives an objective lens 1709 via a tracking actuator based on a tracking error signal TE output from the tracking error signal detector.
In FIG. 17, light reflected from an optical recording medium 1702 that is rotated by a spindle motor 1701 is detected by a photodetector 1703. A detection signal of the photodetector 1703 is input to the amplifier 1704, which then produces a summation signal RFA1. The summation signal RFA1 is input to the PLL circuit 1705 and the S-H circuit 1707.
FIG. 18 shows the arrangement of marks formed on the optical recording medium 1702. In FIG. 18, a horizontal axis represents the circumferential direction of the optical recording medium 1702. A clock mark 1802 is located on an imaginary track centerline 1801. A first wobble mark 1803 is located at a certain distance away from the track centerline 1801 on the outer circumference side, while a second wobble mark 1804 is located at a certain distance away from the track centerline 1801 on the inner circumference side. When a light spot passes along the track centerline 1801, an amount of light reflected from the first wobble mark 1803 is equal to that from the second wobble mark 1804. When the light spot passes along the outer circumference side of the track centerline, the amount of light reflected from the second wobble mark 1804 is smaller than that from the first wobble mark 1803. When the light spot passes along the inner circumference side of the track centerline, the amount of light reflected from the second wobble mark 1804 is larger than that from the first wobble mark 1803. By comparing the reflected light from the first wobble mark 1803 with that from the second wobble mark 1804, a tracking error signal, which indicates the deviation of a light spot from the track centerline 1801, can be detected.
The PLL circuit 1705 in FIG. 17 produces a reproduction clock signal in synchronization with light reflected from the clock mark 1802. The reproduction clock signal is input to the timing generation circuit 1706, which then outputs a timing signal that indicates the reproduction timing for each of the first and second wobble marks 1803, 1804. The S-H circuit 1707 samples and holds the summation signal RFA1 in response to the timing signal, subtracts the summation signal RFA corresponding to the second wobble mark 1804 from that corresponding to the first wobble mark 1803, and outputs the resultant value as a tracking error signal TE.
FIG. 19 shows a waveform in each portion of the tracking error signal detector in FIG. 17. In FIG. 19, a horizontal axis represents time. The summation signal RFA1 from the amplifier 1704 has a reproduction waveform that corresponds to the clock mark 1802, the first wobble mark 1803, and the second wobble mark 1804. The PLL circuit 1705 generates the reproduction clock signal in synchronization with the clock mark 1802 (time A). The timing generation circuit 1706 generates the timing signal in response to the reproduction clock signal from the PLL circuit 1705. The S-H circuit 1707 samples and holds the summation signal RFA1 in accordance with the timing signal. In an example shown in FIG. 19, the summation signal RFA is sampled and held at time B and time C. The sampling value at time B corresponds to the first wobble mark 1803 and the sampling value at time C corresponds to the second wobble mark 1804.
The tracking error signal TE output from the tracking error signal detector having the above configuration is input to the tracking control device 1708 in FIG. 17. The tracking control device 1708 performs a predetermined control operation and drives the tracking actuator. The tracking actuator drives the objective lens 1709. In this manner, a light spot is controlled so as to be on the track centerline 1801. The tracking error signal detector as described above is disclosed, e.g., in JP 6(1994)-60408 A.
In the above configuration, however, it is difficult for the PLL circuit to be pulled in, e.g., when the rotation rate of the optical recording medium changes greatly or sharply. If the PLL circuit is not pulled in, the summation signal RFA1 does not synchronize with the clock of the PLL circuit. Consequently, the timing signal of the timing generation circuit also becomes asynchronous with the summation signal RFA1.
When the summation signal RFA1 does not synchronize with the timing signal, the summation signal RFA1 that corresponds to the first and second wobble marks cannot be detected correctly, making it impossible to detect an accurate tracking error signal TE. This increases tracking errors, which lead to inaccurate recording/reproducing operations on the optical recording medium.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a tracking error signal detector that can produce a high-accuracy tracking error signal even if a summation signal is sampled at asynchronous timing.
To achieve the above object, a tracking error signal detector of the present invention includes a photodetector, an amplifier, a sampling portion, a first arithmetic portion, and a second arithmetic portion. The photodetector detects reflected light from an optical recording medium including a servo area provided with a first wobble mark and a second wobble mark. The amplifier produces a summation signal of the reflected light from a detection signal of the photodetector. The sampling portion samples the summation signal. The first arithmetic portion calculates extremum of the summation signal in the vicinity of reproduction timing for each of the first wobble mark and the second wobble mark from sampling values produced by the sampling portion so as to produce a first extremum and a second extremum. The second arithmetic portion produces a tracking error signal that corresponds to a difference between the first extremum and the second extremum.
This configuration detects the extremum of each reproduction wobble mark by operations with the sampling values of the summation signal in the vicinity of the wobble mark. Therefore, an optimum value always can be obtained, resulting in a high-accuracy tracking error signal.
The first arithmetic portion may include a maximum/minimum sampling value detecting portion, a first estimating portion, and a second estimating portion. The maximum/minimum sampling value detecting portion produces a maximum/minimum of the sampling values in the vicinity of reproduction timing for each of the first wobble mark and the second wobble mark. The first estimating portion estimates a timing of a relative maximum/minimum of the summation signal in the vicinity of reproduction timing for each of the first wobble mark and the second wobble mark by using the maximum/minimum sampling value and its preceding and following sampling values. The second estimating portion estimates a relative maximum/minimum of the summation signal in the vicinity of reproduction timing for each of the first wobble mark and the second wobble mark by using the maximum/minimum sampling value, its preceding and following sampling values, and the timing of the relative maximum/minimum from the first estimating portion.
The first arithmetic portion may include a maximum/minimum sampling value detecting portion and a third estimating portion. The maximum/minimum detecting portion produces a maximum/minimum of the sampling values in the vicinity of reproduction timing for each of the first wobble mark and the second wobble mark. The third estimating portion estimates a relative maximum/minimum of the summation signal in the vicinity of reproduction timing for each of the first wobble mark and the second wobble mark by using the maximum/minimum sampling value and its preceding and following sampling values.
It is preferable that the second arithmetic portion includes an output selecting portion. The output selecting portion selects one of a value corresponding to the difference between the first extremum and the second extremum and a tracking error signal detected from the preceding servo area in accordance with the state of the sampling values so as to output the tracking error signal.
It is preferable that the second arithmetic portion includes a comparing portion and an output selecting portion. The comparing portion compares the first extremum and the second extremum with a sampling value at predetermined timing. The output selecting portion selects one of a value corresponding to the difference between the first extremum and the second extremum and a tracking error signal detected from the preceding servo area in accordance with the comparison result of the comparing portion so as to output the tracking error signal.
It is preferable that the second arithmetic portion includes a portion for comparing sampling values between marks and an output selecting portion. The portion for comparing sampling values between marks compares the sampling values of the summation signal in the vicinity of reproduction timing between the first wobble mark and the second wobble mark with a predetermined value. The output selecting portion selects one of a value corresponding to the difference between the first extremum and the second extremum and a tracking error signal detected from the preceding servo area in accordance with the comparison result of the portion for comparing sampling values between marks so as to output the tracking error signal.
It is preferable that the output selecting portion changes a selecting/outputting operation in accordance with the operating state of tracking control.
It is preferable that the second arithmetic portion selects one of a value corresponding to the difference between the first extremum and the second extremum and a tracking error signal detected from the preceding servo area in accordance with directions in which the first extremum and the second extremum change during the tracking control operation so as to output the tracking error signal.